Recently, thermoplastic acrylonitrile-butadiene-styrene (hereinafter ABS) resin has been employed to materials for electric/electronic appliances, automobile parts and so on. The requirements which the materials have to meet include heat resistance, mechanical strength and dimensional stability. In particular, when the resin is molded through extrusion or blow molding process, the materials further require high melt modulus and elongation ratio and should not cause a phenomenon of draw-down of parrison.
It has been proposed to increase the amount of rubber content to improve the impact resistance of ABS resin. However, the molding compositions produced therefrom have inordinately high coefficient of linear thermal expansion, so that the dimensional stability of the resin is deteriorated. On the other hand, decreasing the amount of rubber content may cause crack formation due to poor impact resistance at low temperature. Inorganic fillers such as glass fiber may be used to improve dimensional stability, but this method is inadequate for the resins to be prepared by extrusion or blow molding process.
In order to improve impact strength, it is also known to blend polymer particles produced by different polymerization process, that is, to blend polymer particles produced in an emulsion polymerization process with particles produced in a continuous polymerization process (c.f. U.S. Pat. Nos. 4,430,478, 5,605,963 and 5,696,204). However, the resultant products exhibit low dimensional stability and poor extrusion and blow molding properties.
That is, in order to obtain good extrusion and blow molding property, it is important that the resin compositions have high melt modulus and elongation ratio, and exhibit high strain hardening effect and no parrison phenomenon occurs. However, the exacting requirements are not fulfilled by these compositions produced from the above references.
It is commonly known a method to minimize the rubber content and add N-substituted maleimide copolymer in order to provide good characteristics of heat resistance and dimensional stability to ABS resin. Though N-substituted maleimide copolymer confers improved heat resistance and low coefficient of linear thermal expansion, the use of N-substituted maleimide cannot provide sufficient impact resistance, so the application has been restricted.
Accordingly, the present inventors have developed a thermoplastic ABS resin composition having improved impact resistance, dimensional stability and heat resistance as well as excellent extrusion and blow molding property by introducing ABS copolymer particles having different rubber particle size to vinyl cyanide-vinyl aromatic copolymer in a specific ratio.